5 ms·

Can anyone shed some light on how this new setup will be more difficult to jam?

by politelemon 8y ago

cwt137 8y ago
The article said the signal is stronger. That is the new anti-jamming countermeasure.
- tialaramex 8y ago
  Just broadcasting a similarly strong signal won't jam GPS family transmissions. This is because the transmission is based on a pseudorandom bit sequence. It looks like noise unless you also know what the pseudorandom sequence should be. If you do you can pick this out from any not related signal trying to jam it, until the jammers pour in so much more power that it overwhelms your receiver. The new birds know a pseudorandom sequence that was state of the art ten years ago rather than forty years ago (times approximate). This is a shared key system. If you either break the system or know the key, you get position data or can effectively jam it. You can put several transmissions on the same bird, and give away some of the keys, voila you're a public service. Keep one or more for your military and they've got position information that can't be jammed.
  - cameldrv 8y ago
    Anything can be jammed, you just need enough power. Having a secret spread spectrum code helps. It's the same principle but easier to understand in the frequency hopping case. In that case, suppose you want as much jamming power as signal power, you have to spread it out across the whole band because you don't know where the next frequency hop is going to be. So if say the system uses 1/100 of the available bandwidth at any one time, now the jammer needs to be 100x as powerful to jam the whole band with the same power as the jammed system. Still, the satellite puts out a low power signal from thousands of miles away, so the jammer starts with an advantage. Where the new block iii satellites will improve this is with a spot beam that can be directed at a particular area on earth where the jamming is expected, and locally get about 100x the power. I suspect that you'll see more directional antennas and phased arrays applied to GPS receivers as well, since the angular direction to the satellite is known, and it's difficult to put the jammer right in the path of between the receiver and the satellite.
dr_orpheus 8y ago
The new block of GPS satellites contain a new acquisition code called M-code. It is an encrypted code from the GPS satellites that allows military receivers to still be able to perform positioning if the regular coarse acquisition (C/A) code (what we all use) is being jammed. In the current GPS setup there is already an encrypted code (called P-code) but it often requires the GPS receiver first acquiring on the C/A code before being able to track solely on the P code. By contrast, military GPS receivers will be able to lock directly on to the M-code without any coarse acquisition. https://en.wikipedia.org/wiki/GPS_signals#Military_(M-code) https://en.wikipedia.org/wiki/GPS_signals#Military_(M-code) https://www.gpsworld.com/the-promises-of-m-code-and-quantum/ https://www.gpsworld.com/the-promises-of-m-code-and-quantum/
michaelt 8y ago
1. Adds a new directional 'spot beam' that can provide a 20dB signal boost (100x power boost) in a target region several hundred kilometres in diameter. 2. A new(-ish) military signal, the 'M Code', which is present on some but not all satellites from Block II. It has a number of features including [1]: * Secretive design. * Much wider bandwidth than civilian C/A code, meaning wideband jamming needs more energy. * Power distribution different to the civilian C/A code, and unlike the older P(Y) code doesn't need the receiver to acquire the C/A code first, so less impacted if someone jams the civilian C/A code. * A more modern spread-spectrum design (from 2000 rather than 1973) that provides better noise immunity and forward error correction, and hence better jamming immunity. * The parts of the spread-spectrum design that rely on the transmitter and receiver having a common source of pseudorandom data use more modern cryptography. [1] https://www.mitre.org/sites/default/files/pdf/betz_overview.pdf https://www.mitre.org/sites/default/files/pdf/betz_overview....
- toomuchtodo 8y ago
  Interesting that the spot beam is not phased array, but a parabolic dish on the satellite body.
  - AWildC182 8y ago
    GPS signals are sufficiently low frequency that a phased array would need to be fairly large. Each antenna in the array would need to be around 5cm and you'd need quite a few of them on each axis. Totally doable but someone probably decided a dish was easier/cheaper than a big board of antennas or waveguides and associated circuitry. Phased array would be nice to have though, since if the array was large enough you could have multiple beams produced by different parts of the array and cover more/multiple areas.

Can anyone shed some light on how this new setup will be more difficult to jam?

cwt137 8y ago

The article said the signal is stronger. That is the new anti-jamming countermeasure.

tialaramex 8y ago
Just broadcasting a similarly strong signal won't jam GPS family transmissions. This is because the transmission is based on a pseudorandom bit sequence. It looks like noise unless you also know what the pseudorandom sequence should be. If you do you can pick this out from any not related signal trying to jam it, until the jammers pour in so much more power that it overwhelms your receiver. The new birds know a pseudorandom sequence that was state of the art ten years ago rather than forty years ago (times approximate). This is a shared key system. If you either break the system or know the key, you get position data or can effectively jam it. You can put several transmissions on the same bird, and give away some of the keys, voila you're a public service. Keep one or more for your military and they've got position information that can't be jammed.
- cameldrv 8y ago
  Anything can be jammed, you just need enough power. Having a secret spread spectrum code helps. It's the same principle but easier to understand in the frequency hopping case. In that case, suppose you want as much jamming power as signal power, you have to spread it out across the whole band because you don't know where the next frequency hop is going to be. So if say the system uses 1/100 of the available bandwidth at any one time, now the jammer needs to be 100x as powerful to jam the whole band with the same power as the jammed system. Still, the satellite puts out a low power signal from thousands of miles away, so the jammer starts with an advantage. Where the new block iii satellites will improve this is with a spot beam that can be directed at a particular area on earth where the jamming is expected, and locally get about 100x the power. I suspect that you'll see more directional antennas and phased arrays applied to GPS receivers as well, since the angular direction to the satellite is known, and it's difficult to put the jammer right in the path of between the receiver and the satellite.

dr_orpheus 8y ago

The new block of GPS satellites contain a new acquisition code called M-code. It is an encrypted code from the GPS satellites that allows military receivers to still be able to perform positioning if the regular coarse acquisition (C/A) code (what we all use) is being jammed. In the current GPS setup there is already an encrypted code (called P-code) but it often requires the GPS receiver first acquiring on the C/A code before being able to track solely on the P code. By contrast, military GPS receivers will be able to lock directly on to the M-code without any coarse acquisition. https://en.wikipedia.org/wiki/GPS_signals#Military_(M-code) https://en.wikipedia.org/wiki/GPS_signals#Military_(M-code) https://www.gpsworld.com/the-promises-of-m-code-and-quantum/ https://www.gpsworld.com/the-promises-of-m-code-and-quantum/

michaelt 8y ago

1. Adds a new directional 'spot beam' that can provide a 20dB signal boost (100x power boost) in a target region several hundred kilometres in diameter. 2. A new(-ish) military signal, the 'M Code', which is present on some but not all satellites from Block II. It has a number of features including [1]: * Secretive design. * Much wider bandwidth than civilian C/A code, meaning wideband jamming needs more energy. * Power distribution different to the civilian C/A code, and unlike the older P(Y) code doesn't need the receiver to acquire the C/A code first, so less impacted if someone jams the civilian C/A code. * A more modern spread-spectrum design (from 2000 rather than 1973) that provides better noise immunity and forward error correction, and hence better jamming immunity. * The parts of the spread-spectrum design that rely on the transmitter and receiver having a common source of pseudorandom data use more modern cryptography. [1] https://www.mitre.org/sites/default/files/pdf/betz_overview.pdf https://www.mitre.org/sites/default/files/pdf/betz_overview....

toomuchtodo 8y ago
Interesting that the spot beam is not phased array, but a parabolic dish on the satellite body.
- AWildC182 8y ago
  GPS signals are sufficiently low frequency that a phased array would need to be fairly large. Each antenna in the array would need to be around 5cm and you'd need quite a few of them on each axis. Totally doable but someone probably decided a dish was easier/cheaper than a big board of antennas or waveguides and associated circuitry. Phased array would be nice to have though, since if the array was large enough you could have multiple beams produced by different parts of the array and cover more/multiple areas.